The invention relates to a method and an apparatus for controlling the regeneration of an NOx storage converter that is disposed in the exhaust-gas system of an internal-combustion engine and can be operated in an absorption mode and a regeneration mode, with the operating parameters of the internal-combustion engine being changed as a function of the operating state of the NOx storage converter.
NOx storage converters are primarily used in lean-mix engines. In this type of engine, the so-called lean-mix operation, with a so-called lambda value greater than 1 of the air-fuel mixture, is preferred over a stoichiometric operation, with a lambda value=1, or a so-called rich-mix operation, with a lambda value less than 1, because a significantly lower fuel consumption can be attained with a surplus of air in the air-fuel mixture, i.e., with a lambda value greater than 1. Stratified-charge engines represent a special kind of lean-mix motor. In the stratified-charge operation of a stratified-charge engine, a lean air-fuel mixture is supplied to the engine, and an ignitable, rich air-fuel mixture is made available in the combustion chamber in the vicinity of the spark plug, while a lean mixture is present in the remainder of the combustion chamber. The spark plug first ignites the rich air-fuel mixture, which then ignites the lean mixture. The 3-way converter conventionally used up to this point, however, cannot single-handedly reduce the exhaust gases that are formed in the operation of a lean-mix engine to harmless gas components, because it requires the supply of air and fuel in a precisely-defined stoichiometric ratio. An NOx converter, in contrast, can absorptively store nitrogen oxides for a limited time under certain marginal conditions, with a lambda value greater than 1, and re-release them at a later time, with a lambda value less than 1 or =1, and reduce them to harmless gases. In a stratified-charge engine, the NOx converter is operated in a so-called absorption mode during stratified-charge operation. In the homogeneous operation of the engine, in contrast, a stoichiometric or rich air-fuel mixture is supplied, and the NOx storage converter is operated in a so-called regeneration mode. Usually, NOx storage converters operate in storage cycles, which encompass at least one relatively slow absorption mode and a faster regeneration mode.
The function and efficiency of an NOx storage converter depend on numerous influential factors, and can particularly be subjected to reversible and irreversible damage. Reversible damage can be caused by, for example, a thionation of the converter, which notably leads to a reduction in the NOx storage capacity or the creation of mechanical stresses in the converter. Thermal damage, such as the sintering of a converter component, the separation of converter and storage components, or an increasingly inhomogeneous, near-surface NOx charge, cause irreversible damage to the NOx converter. Thermal damage typically results in not only a reduction in the NOx storage capacity, but also a reduced oxygen storage capacity of the converter. Production-related variations in properties, along with these operation-related types of damage, can also influence the efficiency and function of the converters. Under certain marginal conditions, reversible damage to an NOx converter can be at least partially remedied through regeneration measures during driving operation. For example, desulfurization can be achieved through a temporary increase in the exhaust-gas temperature. With the occurrence of irreversible damage, however, the only possible regeneration measure is to adapt the operating parameters of the engine and/or the exhaust-gas system, thereby taking into account the altered efficiency of the exhaust-gas purification.
DE 196 07 151 C1 discloses a method for regenerating an NOx storage converter, in which a regeneration phase is initiated as a function of an operating state of the NOx storage converter. The operating state corresponds to at least one limit quantity of NOx compounds that are emitted by the NOx storage converter. The emitted quantity of NOx compounds is ascertained from the signal of a lambda sensor disposed upstream of the NOx storage converter. It is impossible, however, to reliably ascertain the operating state of the NOx storage converter, particularly the degree of damage, so the control of the regeneration is correspondingly imprecise.
EP 0936349 A2 discloses a system for diagnosing an NOx converter that is connected to an internal-combustion engine, and in which the signals of an NOx-sensitive sensor disposed behind the converter are evaluated for assessing the extent of the damage. This document does not, however, describe a control of the regeneration of the NOx storage converter. In this connection, EP 0936349 A2 further discloses a reduction in the NOx concentration after the switch to a rich air-fuel mixture. The NOx concentration reaches a minimum after a certain time in order to increase subsequently to higher values, and to finally attain a value again that it had attained prior to the switch to an oxygen deficiency. In the known system, the state of the NOx storage converter, or the damage thereto, is ascertained from the rate of change in the NOx concentration after the minimum has been reached. This requires the use of values of the NOx concentration within a relatively large time interval after the switch to an oxygen deficiency, which results in a correspondingly long diagnosis period. A further drawback is that the rate of change of the NOx concentration in the used time interval is a function of the operating parameters of the engine and the exhaust-gas system, and therefore requires complex corrective measures.
It is the object of the invention to provide a method and an apparatus for controlling the regeneration of an NOx storage converter that is essentially based on the assessment of values of the NOx concentration within a relatively short time interval, and a relatively fast, simple determination of the operating state of the NOx storage converter for achieving an optimum regeneration.
This object is accomplished with the features of the independent claims.
The invention is based on the realization that, when the NOx storage converter switches from an absorption mode to a regeneration mode within a short time interval, only a portion of the released NOx is catalytically converted. The unconverted portion of the NOx causes a temporary increase in the NOx concentration in the exhaust gas, the so-called desorption peak. Characteristic properties of this peak, such as duration, height or the like, relate to the function of, or, if applicable, the damage to the NOx converter. In accordance with the invention, the NOx concentration in the exhaust gas is measured downstream of the NOx storage converter, and for determining the operating state of the NOx storage converter in a transition of the NOx storage converter from the absorption mode to the regeneration mode, the values of characteristic features of an NOx desorption peak are ascertained in the time curve of the NOx concentration, then compared to predetermined test patterns; in the process, a comparison result is formed, and a converter-state signal that characterizes the operating state of the NOx storage converter is derived from the comparison result. Depending on the converter-state signal, a change is made to the operating parameters, which includes implementing a regeneration measure for attaining an optimum regeneration of the NOx storage converter. Because the NOx desorption peak occurs within a relatively short time interval, for example after the transition from a lean to a rich or stoichiometric air-fuel mixture, it is possible to ascertain the operating state in a relatively short time interval. In an ideal case, the duration of a single NOx desorption peak is sufficient. The ascertainment of values of characteristic features of the NOx desorption peak in accordance with the invention permits an especially simple evaluation of the time curve of the NOx concentration in the time interval of concern, and therefore only requires a small outlay for identification.
Further features and advantages of the present invention ensue from the dependent claims and, independently of their summary in the claims, from the following description of preferred exemplary embodiments according to the invention, in conjunction with the associated drawings.